In order to continue improving the spectral efficiency of GSM/EDGE Global (System for Mobile Communication (GSM)/Enhanced Data for GSM Evolution), there is a need to find effective ways of multiplexing multiple users' signals into a single time slot over a 200 kHz channel while avoiding or minimizing mutual interference among users.
In accordance with a proposal in 3GPP/GERAN (third generation partnership project/GSM EDGE Radio Access Network) a fast feedback channel for Voice over IP is proposed, see GERAN#44 GP-091988 “Fast Feedback Channel” (v1). The idea is to allow the Voice over internet protocol (IP) users to signal the network when there are packets available for transmission. This channel must allow fast feedback and must consume as little bandwidth as possible. To this end, several users are multiplexed into one timeslot by means of time division multiplexing.
Another example is Voice services over Adaptive Multi-user channels on One Slot (VAMOS), in which up to two full rate users are multiplexed in the same time slot. VAMOS is a standardized 3GPP/GSM feature. However, in the uplink, the receiver requires complex multi-user detection since the two user signals are ordinary Gaussian minimum shift keying (GMSK) co-channel interferers.
Due to the scarcity of the electromagnetic spectrum, it is desirable to multiplex more than 2 users into a single time slot, while keeping the receiver complexity low. Moreover, it is desirable to allow the users to employ GMSK modulation. This non-linear modulation technique has very good spectral properties and it is well suited for energy efficient analog Radio Frequency (RF) front ends. Energy efficiency is always important in mobile stations.
Multi-user multiplexing has been standardized in GSM. In the uplink the users transmit using GMSK modulation, and become ordinary co-channel interferers. In the downlink AQPSK modulation is used, see 3GPP Technical Specification TS 45.004 v9.1.0. Note that neither the uplink nor the downlink transmission modes in VAMOS create truly orthogonal sub-channels. In the uplink the users are ordinary co-channel interferers, and the signals are separated in the receiver with the help of their training sequences. The training sequences are such that they have low cross correlation.
Two-layer transmission was standardized recently in GSM (Global System for Mobile communication). The feature is as set out above called VAMOS. Since accurate synchronization and channel estimation are critical for successful decoding, a new set of training sequences was also introduced, see 3GPP TS 45.002 v9.5.0. These training sequences were designed to be orthogonal with the legacy set of training sequences, in the sense that the cross-correlations are small. However, complete orthogonality is not achieved. It seems difficult to increase the number of layers, say to 3 or 4 users, and still be able to perform accurate channel estimation. This is particularly challenging if the receiver has a small number of antennas, say one or two. As an illustration, the Cramer-Rao lower bound of the estimation error for a 5-tap channel estimate is shown in FIGS. 1-3, assuming a single antenna receiver. This is a theoretical bound on the variance of the estimation error, see Steven M. Kay, “Fundamentals of Statistical Signal Processing, Estimation Theory”, Prentice Hall 1993. Theorem 4.1. Training sequences 0, 3 of set 1 and set 2, in 3GPP TS 45.002 v9.5.0, see tables 5.2.3a and 5.2.3b, have been chosen due to their good cross correlation properties. FIG. 2 shows the power of the estimation error for one user, normalized so that the maximum error has 0 dB variance. FIG. 2 shows the variance of the estimation error for two users. Now there are twice as many taps to estimate, so that the estimation error increases. FIG. 3 shows the variance of the estimation error for 4 users. Not surprisingly, the theoretical error has increased dramatically, since 20=4×5 parameters must be estimated given the same number of received samples. Therefore, it is doubtful whether a single antenna receiver can estimate with sufficient accuracy the channels for 4 simultaneous users. In other words, a straightforward extension of the VAMOS technique in the uplink to more than two users seems unfeasible. There is therefore a need for a technique that can handle more than two users in one slot in the uplink of a radio communication system.
In sum there exists a need for an improved transmission methods and devices for use in radio networks and in particular in radio networks where multiple users are to transmit simultaneously in a single transmission slot.